Magnetic recording hard disk drives use a thin-film inductive write head supported on the end of a rotary actuator arm to record data in the recording layer of a rotating disk. The write head is patterned on the trailing surface of a head carrier, such as a slider with an air-bearing surface (ABS) that allows the slider to ride on a thin film of air above the surface of the rotating disk. The write head is an inductive head with a thin film electrical coil located between the poles of a magnetic yoke. When write current is applied to the coil, the pole tips provide a localized magnetic field across a gap that magnetizes regions of the recording layer on the disk so that the magnetic moments of the magnetized regions are oriented into one of two distinct directions. The transitions between the magnetized regions represent the two magnetic states or binary data bits. Commercially-available disk drives use horizontal or longitudinal recording wherein the magnetic moments of the magnetized regions are oriented “longitudinally” in the plane of the recording layer.
The magnetic material (or media) for the recording layer on the disk is chosen to have sufficient coercivity such that the magnetized data bits are written precisely and retain their magnetization state until written over by new data bits. The data bits are written in a sequence of magnetization states to store binary information in the drive and the recorded information is read back with a use of a read head that senses the stray magnetic fields generated from the recorded data bits. Magnetoresistive (MR) read heads include those based on anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) such as the spin-valve type of GMR head, and more recently magnetic tunneling, such as the magnetic tunnel junction (MTJ) head. Both the write and read heads are kept in close proximity to the disk surface by the slider's ABS, which is designed so that the slider “flies” over the disk surface as the disk rotates beneath the slider. In more recently proposed disk drives the write head or a pad supporting the write head may be in physical contact with the disk so that there is no air-bearing in the region of the write head.
The areal data density (the number of bits that can be recorded on a unit surface area of the disk) is now approaching the point where magnetic grains that make up the data bits are so small that they can be demagnetized simply from thermal instability or agitation within the magnetized bit (the so-called “superparamagnetic” effect). To avoid thermal instabilities of the stored magnetization, a minimal stability ratio of stored magnetic energy per grain, KUV, to thermal energy, kBT, of KUV/kBT >>60 will be required, where KU and V are the magneto-crystalline anisotropy and the magnetic switching volume, respectively, and kB and T are the Boltzmann constant and absolute temperature, respectively. Because a minimum number of grains of magnetic material per bit are required to prevent unacceptable media noise, the switching volume V will have to decrease, and accordingly KU will have to increase to further shrink bit sizes. However, increasing KU also increases the switching field, H0, which is proportional to the ratio KU/MS, where MS is the saturation magnetization (the magnetic moment per unit volume). (The switching field H0 is the field required to reverse the magnetization direction, which for most magnetic materials is very close to but slightly greater than the coercivity or coercive field HC of the material.) Obviously, H0 cannot exceed the write field capability of the recording head, which currently is limited to about 9 kOe for longitudinal recording, and perhaps 15 kOe for perpendicular recording.
Since it is known that the coercivity of the magnetic material of the recording layer is temperature dependent, one proposed solution to the thermal stability problem is thermally-assisted magnetic recording (TAMR), wherein the magnetic material is heated locally to near or above its Curie temperature during writing to lower the coercivity enough for writing to occur, but where the coercivity/anisotropy is high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature). Several TAMR approaches have been proposed, primarily for longitudinal recording.
A “wide-area” heater has been proposed to heat a region of the disk wider than the data track to be recorded. A wide-area heater is relatively easy to implement in a conventional recording head structure and has the additional advantage that it heats the data track very efficiently and thus minimizes the required heater temperature for a given required media temperature. TAMR systems with wide-area heaters include systems that use a laser or ultraviolet lamp to do the heating, as described in “Data Recording at Ultra High Density”, IBM Technical Disclosure Bulletin, Vol. 39, No. 7, July 1996, p. 237; “Thermally-Assisted Magnetic Recording”, IBM Technical Disclosure Bulletin, Vol. 40, No. 10, October 1997, p. 65; and U.S. Pat. Nos. 5,583,727 and 5,986,978. One problem with a wide-area heater is adjacent-track interference (ATI). Because adjacent tracks are also being heated, the stray magnetic field from the write head can erase data previously recorded in the adjacent tracks. Also, even in the absence of a magnetic field, the thermal decay rate in adjacent tracks is accelerated over that at ambient temperature and thus data loss may occur.
A proposed solution for the ATI problem is a “small-area” heater that heats only the data track. U.S. Pat. No. 6,493,183 describes a TAMR disk drive, also for longitudinal recording, wherein the write head includes an electrically resistive heater located in the write gap between the pole tips for locally heating just the data track. A disadvantage of the small-area resistive heater is that due to the relatively inefficient heat transfer the heater temperatures required to reach a desired media temperature are very high.
While until now only longitudinal magnetic recording has been successfully commercialized, perpendicular magnetic recording has been widely studied and suggested as a promising path toward ultra-high recording densities. In perpendicular recording, the magnetic moments of the magnetized regions are oriented perpendicular to the plane of the recording layer. The most common type of perpendicular magnetic recording system is one that uses a “probe” or single pole type (SPT) write head with a “dual-layer” magnetic recording disk. The dual-layer disk has a perpendicular magnetic recording data layer formed on a magnetically “soft” or relatively low-coercivity magnetically permeable underlayer, the underlayer serving as a flux return path for the write field from the SPT head.
Perpendicular magnetic recording provides the advantage that the magnetically soft, permeable underlayer effectively increases the write field available from the write pole, thus allowing the use of magnetic recording media with a higher coercivity/anisotropy and thus higher stability against magnetization decay. Ultimately, however, perpendicular magnetic recording is also limited by the superparamagnetic effect.
To improve storage technology and specifically the areal bit density beyond the limitations of longitudinal and perpendicular recording, a TAMR system, and in particular a TAMR head, is needed that solves the ATI problem and provides the additional advantages of perpendicular magnetic recording.